1. Field of the Invention
The present invention relates to a composition for forming an electrode active material of a lithium secondary battery, a composition for forming a separator and a method of preparing a lithium secondary battery using the compositions, and more particularly, a composition for forming an electrode active material of a simplified lithium secondary battery without a step of extracting a plasticizer using an organic solvent, a composition for forming a separator and a method of preparing a lithium secondary battery using the compositions.
2. Description of the Related Art
According to the kind of electrolyte used, lithium secondary batteries are classified into lithium ion batteries using liquid electrolyte and lithium ion polymer battery using polymer solid electrolyte.
Among lithium secondary batteries, lithium ion batteries have been successfully commercialized by Sony Corp. in 1991. Also, consistent attempts to commercialization of lithium ion polymer batteries have been made since they were developed by Bell Communications Research Inc. in 1995. Lithium ion polymer batteries which are in widespread use, polyvinylidenefluoride resin that absorbs a non-aqueous electrolytic solution to have excellent ionic conductivity, is used as solid electrolyte.
In manufacturing a lithium ion polymer battery, during formation of an electrode and polymer electrolyte, poor interlayer adhesion between the electrode and polymer electrolyte increases internal resistance of the battery, thereby considerably degrading the performance of battery. Thus, in order to improve the characteristics of a lithium ion polymer battery, it is essential to improve the interlayer adhesion between an electrode and polymer electrolyte.
In order to enhance the interlayer adhesion between an electrode and polymer electrolyte, Bell Communications Research, Inc. has proposed a method in which an electrode binder, vinylidenefluoride (VdF)-hexafluoropropylene (HFP) copolymer as a polymer resin of solid electrolyte, wherein the unit of HFP is 8 to 25%, was used, and a separately fabricated electrode and solid polymer electrolyte were thermally laminated at a temperature of 100xc2x0 C. or higher to be adhered to each other (U.S. Pat. Nos. 5,460,904, 5,540,741 and 5,607,485). As a plasticizer for forming pores in the electrode and the polymer electrolyte, dibutyl phthalate (DBP) was used. DBP is generally removed by being repeatedly extracted using an organic solvent such as ether or methanol.
However, removal of a plasticizer by extraction using an organic solvent requires much time, thereby lowering the manufacturing efficiency of batteries. Also, a solvent used in extracting a plasticizer and an equipment for extracting the plasticizer increase the manufacturing cost. Further, a system for recovering the solvent used for plasticizer extraction is necessary.
To solve the above problems, it is a first object of the present invention to provide a composition for forming an electrode active material of a lithium secondary battery which does not require a plasticizer extraction process using an organic solvent.
It is a second object of the present invention to provide a composition for forming a separator of a lithium secondary battery which does not require a plasticizer extraction process using an organic solvent.
It is a third object of the present invention to provide a method of preparing a simplified lithium secondary battery which can reduce the manufacturing cost and time, and which has excellent lifetime characteristic, high performance and low temperature characteristic.
To achieve the first object of the present invention, there is provided a composition for forming an electrode active material of a lithium secondary battery, having an electrode active material, a plasticizer, a binder and a solvent, wherein the plasticizer is at least one material selected from the group consisting of an ethylene glycol derivative, a cyclic carbonate, a non-cyclic carbonate and propylene glycol carbonate.
The composition may further include a conductive agent. Also, it is preferable that the ethylene glycol derivative is at least one material selected from the group consisting of ethylene glycol diacetate, ethylene glycol dibutyrate, ethylene glycol dibutylether and ethylene glycol dipropionate; the cyclic carbonate is at least one material selected from the group consisting of ethylene carbonate and propylene carbonate; the noncyclic carbonate has a boiling point of 70 to 130xc2x0 C. and is at least one material selected from the group consisting of diethyl carbonate, dimethyl carbonate and ethylmethyl carbonate; and the propylene glycol derivative has a boiling point of 120 to 160xc2x0 C. and is at least one material selected from the group consisting of propylene glycol methyl ether acetate, propylene glycol monomethylether and propylene glycol monoethyl ether.
To achieve the second object of the present invention, there is provided a composition for forming a separator of a lithium secondary battery, having a polymer resin, a plasticizer, a filler and a solvent, wherein the plasticizer is at least one material selected from the group consisting of an ethylene glycol derivative, a cyclic carbonate, a non-cyclic carbonate and propylene glycol carbonate.
Preferably, the polymer resin is polyvinylidenefluoride (PVdF), vinylidenefluoride-hexafluoropropylene copolymer in which the content of HFP is greater than 0 and less than 8% by weight (VdF-HFP copolymer A), a mixture of VdF-HFP copolymer A and PVdF, or a mixture of VdF-HFP copolymer A and VdF-HFP copolymer in which the content of HFP is greater than 0 and less than or equal to 15% by weight (VdF-HFP copolymer B).
To achieve the third object of the present invention, there is provided a method for manufacturing a lithium secondary battery including the steps of (a) coating electrode active material compositions each comprising a electrode active material, a binder and a solvent on an electrode current collector to form a cathode and an anode, (b) forming a separator on both surfaces of the anode using a composition for forming a separator comprising a polymer resin, a plasticizer, a filler and a solvent; (c) disposing and fixedly adhering the cathode on the separator to form a battery structure, (d) drying the battery structure under a vacuum condition, and (e) impregnating an electrolytic solution into the resultant structure, wherein the plasticizer of the composition for forming the separator is at least one material selected from the group consisting of an ethylene glycol derivative, a cyclic carbonate, a non-cyclic carbonate and propylene glycol carbonate.